A passive optical network (Passive Optical Network, PON) gradually becomes a mainstream technology in the broadband access field. As various broadband services such as videoconferencing, 3D television, mobile backhaul, and an interactive game develop fast, a requirement on an access bandwidth becomes higher. Facing future evolution of broadband, a broader bandwidth, a longer distance, and larger splitting are required. In a typical PON system, a plurality of terminals (Optical Network Unit, ONU) connects to an optical splitter (Splitter) by using an optical fiber, and, after convergence by an optical splitter, connects to an optical line terminal (Optical Line Terminal OLT) by using a trunk fiber. An ONU shares the trunk fiber by means of TDMA (Time Division Multiple Access, time division multiple access). In a network upgrade process, the ONU shall try to remain unchanged or slightly change at a low cost, so as to protect an existing investment; and an increase in a splitting ratio of an optical splitter means a larger loss in optical power. A universal manner at present for increasing a power budget in the condition of keeping a passive characteristic of the PON includes: introducing a coherent receiving technology, amplifying an optical signal by introducing an optical local oscillator with relatively high power. In addition, an optical receiver works in shot noise dominated status and is capable of reaching a shot noise limit of the receiver, which greatly improves sensitivity. A coherent detection technology applies to any band. However, a coherent receiving technology at present requires precision control over a frequency offset between the optical local oscillator and the optical signal, leading to complexity and an expensive cost of an implementation process of the receiver. Once there is a relatively large error in the frequency offset at the receiver end, a relatively large loss in the optical power is caused.